The afferent and efferent connections of the ventromedial thalamic nucleus in the rat

Abstract

Abstract The afferent and efferent connections of the ventromedial (VM) nucleus of the thalamus in the rat were studied by experiments using the methods of retrograde cell marking by horseradish peroxidase (HRP) and anterograde fiber tracing by autoradiography. Tritiated amino acids deposited microelectrophoretically into VM label a cortical projection that is distributed to a sharply defined superficial portion of layer I of almost the entire extent of the ipsilateral neocortex. The labeling is most dense at frontal cortical levels, where fibers radiate through the deeper layers to terminate in the outer one‐quarter of layer I throughout all neocortex rostral to the genu of the corpus callosum. A lesser number of labeled fibers extends caudally in a supracallosal location to innervate parieto‐occipital cortical areas. Labeled collaterals ascend through the cortical layers to reach layer I, where grains in the superficial portion are found in a gradually decreasing rostrocaudal gradient of density that reaches the caudal pole of the hemisphere. Coronal sections at most levels contain a band of labeling in layer I that extends uninterrupted from the callosal sulcus at the midline to the banks of the rhinal sulcus laterally. Caudal retrosplenial and ventral temporal areas appear to be the only sectors of neocortex spared by the ubiquitous projection. Evidence for additional terminal distribution in deeper layers is found only in the dorsal and lateral sectors of the cortex rostral to the genu where sparsely labeled bands appear in layers III and V. The nearly exclusive distribution of VM's cortical afferents to layer I is compared and contrasted with multilaminar distributions of other “unspecific” cortical afferent fibers. HRP injected into VM labels neurons in a variety of structures at levels ranging from the frontal cortex to caudal medulla. Cell labelling in the globus pallidus, deep layers of the superior colliculus, cerebellar nuclei and the substantia nigra, pars reticulata suggest that VM is a point of convergence for several components of the extrapyramidal motor system. The nigrothalamic projection is topographic: medial and lateral districts of the pars reticulata are connected to medial and lateral districts of VM, respectively. A dorsal‐ventral association may also obtain. Cell labeling in the prefrontal cortex, the cortex along the rhinal sulcus, the lateral habenular nucleus, tegmental and medullary reticular formations, and parabrachial nuclei indicates that VM also receives projections from more heterogeneous sources.